Adjustable height scaffolds are well known in the art and typically comprise four main elements: an upright, a bracket assembly for supporting a work platform on the upright, a jack or block and tackle for raising and lowering the bracket assembly on the upright, and a cage for holding the upper end of the upright in place.
Two types of adjustable height scaffolds, which are designed with uprights, constructed of wood or rubber-backed aluminum are well known and widely used. The load bearing surfaces of these uprights are elastically deformable and hence, when loaded, have cross sections that change, i.e., the jaws of the clamp of these scaffolds indent said surfaces temporarily.
An example of an adjustable height scaffold, which is designed for use with a wooden upright is U.S. Pat. No. 2,216,912 to Hoitsma. The Hoitsma patent discloses an angle bracket to which a jack is coupled, and is commonly referred to as a “pumpjack”. Another example of an adjustable height scaffold that is designed for use on an upright constructed of wood is U.S. Pat. No. 2,342,427 to Riblet. The Riblet patent discloses a bracket assembly which is raised and lowered by block and tackle This adjustable height scaffold has been referred to as the “painter's pole”.
Examples of adjustable height scaffolds, designed for use with aluminum uprights to which a rubber strap has been riveted, include U.S. Pat. No. 4,597,471 to Anderson and U.S. Pat. No. 5,259,478 to Berish et al. It should be noted that the Anderson patent and the Berish patent both use the jaws of the pumpjack mechanism disclosed in the Hoitsma patent. It should also be noted that the jaws of the painter's pole bracket disclosed in Riblet '427 have been adapted for use on rubber-backed aluminum uprights.
U.S. Pat. No. 878,455 to Carter, U.S. Pat. No. 2,801,851 to Meek and U.S. Pat. No. 4,308,934 to Jackson disclose related bracket assemblies. The bracket assemblies disclosed in Carter, Meek and Jackson differ from the bracket assemblies noted above in that the inner jaw of their locks is formed from an extended surface of the associated angle bracket, i.e., they do not pivot with respect to the associated angle bracket. The contact points of said jaws with the upright depend on the straightness of the upright and may be too far apart to provide sufficient friction to support the vertical load. Consequently, they require an outer jaw, in order to supply the vertical force needed to support the load, which indents in some way the upright and so changes, temporarily at least, its cross section.
All of the prior art patents use uprights whose cross sections are altered locally when loaded. The present invention, in contrast, teaches how the molecular, friction force between the jaws of the clamp and the upright may safely support a load without temporarily altering the upright's cross section.
In the Pump Jack, a spring is required to initiate contact of the jaws of the clamp with the upright. In the painter's pole, however, the clamp is coupled to the angle bracket in such a manner that a load placed on the angle bracket exerts a turning moment on the clamp which initiates and maintains contact of the jaws of the clamp with the upright so long as the width of the upright exceeds the design width. A clamp with this property is called a “load activated clamp”.
The outer, load supporting jaw of the pumpjack has a square cross section and is fixed in position in the side members of the clamp to insure that the corner of the square jaw bites into the wooden or rubber surface of the upright when the clamp is supporting a load. When the load is being lowered, however, the clamp is rotated to a horizontal position by the worker. Rotating the clamp to a horizontal position moves a face of the square jaw of the pumpjack so that it is more parallel to the upright. This reduces the vertical, supporting force on the jaw and permits the lowering of the scaffold. With the clamp in a horizontal position, so that a face of the jaw is nearly parallel to the upright, the bracket assembly is lowered.
The jaws of the lock of a painter's pole have smooth, cylindrical surfaces that indent the surface of a wooden upright along the points of a shallow cylinder. As we have seen, the outer jaw of a pumpjack has an edge which bites into the surface of an upright along the corner of a square. Because of the elastically, deformable nature of the wood or rubber used in these uprights, the jaws used in the clamps of said adjustable height scaffolds do not damage the uprights. They do, however, alter the cross section of the upright temporarily. Jaws of this type will be referred to as “edged” jaws because they would contact a hard, flat surface on a single line.